This invention relates to amplification circuits, and it relates especially, though not exclusively, to such circuits as may be used in association with electro chemical cells configured to detect carbon monoxide (CO), thereby to enable a warning as to the presence of the gas to be generated. As is known, the gas is harmful of itself, and it is also a typical product of fire. Thus its sensitive and reliable detection is valuable on at least two fronts.
Electro chemical cell technology is proving to be one of the most competent on which to build CO detectors, but the development and handling of the electrical signals generated by such detectors is associated with difficulty, inter alia because they are run in an amperometric mode, where it is critical (a) to hold the cell electrodes at the correct potential to minimise cross-sensitivities, and (b) to control voltage spikes which, due to the typically capacitive behaviour of such cells would be greatly amplified in duration.
It is an object of the present invention to provide a circuit which addresses the difficulty mentioned above.
According to the invention there is provided an amplification circuit intended for use in association with an electro chemical cell operated in amperometric mode, the circuit comprising an operational amplifier coupled to said cell so as to derive output signals therefrom, and a current mirror circuit having at least one component connected in a feedback path of said operational amplifier.
It has been proposed in GB-A-2307753 to utilise a current mirror circuit in association with an operational amplifier and an electrochemical cell. The circumstances envisaged in that proposal, however, differ substantially from the situation addressed by the present invention. In particular, the electro chemical cell is operated in impedance mode, and the current mirror circuit, which is not connected in the feedback path of the operational amplifier, is instead used in the amplifier's power supply and thus is not part of the path for output signals from the cell.